A drift-tube linac (i.e., drift-tube linear accelerator) designed to accelerate charged particles by utilizing a transverse electric (TE) wave (i.e., H mode) where a current flows in a direction perpendicular to a beam axis (i.e., an axis of a charged particle beam) is known as an H-mode drift-tube linac. In the H-mode drift-tube linac, a large number of drift tubes are arranged in a cavity resonator along the beam axis in the resonator, and a predetermined voltage is applied between respective adjacent ones of the drift tubes so that particles are accelerated according to the voltage (i.e., potential difference) successively every tine the particles pass through each of the drift tubes.
One type of H-mode drift-tube linac using an interdigital H-mode (IH) resonator is known as an IH linac. The IH resonator typically has a structure in which a pair of upper and lower plates, called “ridges”, are mounted (in a vertically opposed arrangement) inside a cylindrical-shaped resonator (although the ridges are not essential to the IH linac). A plurality of drift tubes are alternately attached to the upper and lower ridges through respective stems in such a manner as to be aligned in an axial direction of the resonator. Particles will travel in the axial direction while passing through the drift tubes.
Generally, a linac is equipped with a tuner for adjusting a resonant frequency and a voltage distribution in an entire cavity of a resonator. The finer includes a plurality of fixed tuners and a manually-operated tuner. The fixed tuners are installed in a lateral portion of a wall of a resonator tank and adapted to be used for roughly adjusting the voltage distribution and the resonant frequency. The fixed tuners are immovably welded after the rough adjustment. The manually-operated tuner is adapted to be used for fine adjustment to set a final voltage in the cavity.
During operation of the linac, the resonant frequency is likely to vary due to thermal expansion of the tank and other factor. An auto-tuner is an effective means to compensate or correct such a variation in the resonant frequency caused by temperature change, during the linac operation. Specifically, a slight resonant frequency shift on the side of the tank due to temperature change and other factor is detected by measuring a phase difference between a traveling-wave component in an output of a high-frequency amplifier, and a monitoring signal picked up inside the tank. Then, an automatic frequency control (AFC) circuit performs a calculation based on the measured phase difference to drive the auto-tuner in such a manner as to correct the slight resonant frequency shift.
The H-mode drift-tube linac employs a plurality of tuners, wherein some of the tuners are selectively used as an auto-tuner, and the remaining tuners are used as manually-operated and fixed tuners. The plurality of tuners are arranged in an axial direction of a resonator and along an outer surface of a resonator tank, and each adapted to allow an end member thereof to be inserted into a cavity through a lateral portion of a wall of the resonator tank so as to change a circuit constant of the resonator to modify a resonant frequency or a voltage distribution, as will be described later. Among the tuners, one type configured to change an inductance of the resonator is an inductive tuner, i.e., L tuner, and another type configured to change a capacitance of the resonator is a capacitive tuner, i.e., C tuner.
The auto-tuner is required to have a function of changing only a frequency without changing a voltage distribution. In reality, if one of the tuners is changed in position, a voltage distribution in the entire resonator tends to be changed. It is known that this tendency becomes prominent, particularly, in the IH linac. Thus, two or more of the tuners different in position have to be selectively used as auto-tuners in such a manner that respective voltage changes attributed thereto are cancelled out to keep the voltage distribution from being changed. For this purpose, a three-dimensional electromagnetic field calculation is essential to accurately figure out the voltage distribution, because the voltage distribution in the resonator is dependent on an entire structure of the resonator. However, if the three-dimensional electromagnetic field calculation is performed for each of the tuners while finely changing an insertion amount thereof, to figure out a relationship between a tuner insertion amount and a voltage change, a considerable time has to be spent therefor. As a way for evaluating a combination of tuners suitable for auto-tuners, such a repetitive three-dimensional electromagnetic field calculation is not realistic.
Non-Patent Publication 1: Satoshi YAMADA, et al., “Integrated Report on Construction of Heavy Particle Beam Cancer Therapy Equipment”, May/1995, National institute of Radiological Sciences